1. Field of the Invention
This invention generally relates to remote load control systems and more particularly to such systems including one or more protective load disconnect units suitable for load shedding and maximum demand control.
2. Description of the Prior Art
In remote load control systems, an electric power customer's loads are selectively energized and deenergized by a master load controller connected to each of loads to be controlled. Control signals from the load controller activate a power switching device such as a relay at each load location. It is often desirable in such systems to assure that the control signals are isolated and protected from the electric load power signals. Protection is especially required when the control signals are low level or have small signal strengths such as when generated by and/or are intended for operating low power electronic or other circuit devices. It is further often desirable for the controller to remotely monitor the on or off load status by a simple and efficient load sensing and reporting arrangement. The required operations are desired to be accomplished with simple, reliable, compact circuits which are simply, efficiently and economically manufactured.
In U.S. Pat. No. 3,906,242 a computerized utility peak load shaving system transmits radio control signals from a computer controlled central station to different groups of electric power customers. Receivers at each customer location operate on/off units which control separate power relays connected to a load. Electric load demand sensors at each customer location issue signals to a shut-down control at the central station.
In U.S. Pat. No. 4,021,615 electric appliances are turned off in rooms of a building by frequency signals transmitted from a central exchange through telephone lines to a signal detector associated with each room telephone. The detector operates a power relay to disconnect an associated appliance.
In U.S. Pat. No. 3,964,023 a remote load control system includes a master load controller connected by a pair of control lines to a plurality of slave units connected to loads to be controlled. Four types of control signals include either of two opposite polarities and two levels in each polarity. Each slave controller has an electronic servo circuit having an output circuit optically connected to the AC power load. The control signals provide voltages of a predetermined polarity and level to selectively exceed the thresholds of one or two voltage threshold devices. Photocells in the output load circuits are activated when the thresholds are exceeded. The two control feedlines are capable of generating four different operating conditions at the slave controllers. Neither load status sensing nor reporting of the load status back through the control feedlines is provided.
In U.S. Pat. No. 3,916,183 a power control device turns a lamp load off and on in response to ambient light and predetermined daily time cycles. Output contacts of two thermal current responsive relays are connected in series with the load. A photocell and a time controlled contact device produce current to the relay thermal elements. No remote control or status sensing features are provided.
Accordingly, prior art remote load control systems generally include control of a load by a controller circuit arrangement responsive to a master controller control signal. When status of the load is monitored, a separate sensing circuit arrangement and associated lead connections are required. Thus, several lead wire conductors are required for providing separate command and response for the separate remote control and remote monitoring functions. While it is known to isolate a load control signal from the electric power of a load to be controlled, no prior integral load sensing and disconnect unit provides for receiving a load turn-off control signal and responding with a load status response through a single control terminal that is isolated from both a power switching circuit and the power signal sensing input of a load status response circuit.